This invention relates to a method for forming a solder-bumped circuit trace on a printed circuit board or the like. More particularly, this invention relates to such method comprising reflow of a thin solder plate deposited on a circuit trace to cause the solder to coalesce onto an intersection between linear sections of the trace and form a solder bump.
In the manufacture of a microelectronic package, it is know to utilize a solder bump preformed on a circuit trace of a printed circuit board to attach an electronic component. For example, such solder bumps are employed to form solder bump interconnections for attaching an integrated circuit die mounted on a chip carrier to the board. For this purpose, the board includes a copper circuit trace disposed on a dielectric substrate. To mount the component, a solder bump is attached to a bond pad on the component, typically by placing a preformed microsphere of the solder alloy on the pad and heating to reflow the solder. The bumped component is assembled with the board such that the bump rests against the trace at a predetermined site. The assembly is heated to reflow the solder so that, upon cooling to resolidify the solder, the solder is bonded to the trace as well as the component pad to physically join the component to the board and also to electrically connect the component to the trace for conducting electrical signals to and from the component for processing.
In the formation of such solder bump interconnections, it is found to be advantageous to apply a solder bump to the trace at the attachment site. The solder bump may be formed on the circuit board by applying and reflowing the preformed microsphere in a manner similar to the component. However, in forming bumps on the printed circuit board, it is convenient to apply the solder by electroplating. It is known to electroplate solder onto a circuit trace to facilitate bonding of surface mounted components. However, such practices utilize a generally uniform plate that does not provide a sufficient mass for a solder bump interconnection. Thus, there remains a need for preforming a solder bump on a circuit trace at a designated site and having a mass sufficient for forming a solder bump interconnection.